1. Field of the Invention
The present invention relates to a cutting tool with a coating layer formed on a surface of a substrate.
2. Description of Related Art
In cutting tools and members such as wear resistant members and sliding members, for which wear resistance, sliding properties and fracture resistance are required, the technique of improving wear resistance, sliding properties and fracture resistance are applied by forming various coating layers on the surface of a substrate composed of high hardness sintered body such as WC-based cemented carbide, TiCN-based cermet, diamond or cubic boron nitride crystal (cBN) sintered body, or alternatively ceramics such as alumina or silicon nitride.
Such a coating layer, a TiCN layer and a TiAlN layer are generally widely used, and different coating layers have been developed for purposes of higher wear resistance and improvement of fracture resistance.
For example, Japanese Unexamined Patent Publication No. 7-310171 describes a hard coating on the surface of a cemented carbide base material, in which (TiAl)N is the main component, and part of the metal component thereof is substituted for Si, and part of N (nitrogen) is substituted for B (boron). There is disclosed that the oxidation start temperature of the hard coating can be raised to improve oxidation resistance, thereby improving wear resistance of tools. Japanese Unexamined Patent Publication No. 2006-111915 discloses a hard coating of (TiWSi)N composition, and describes that the adherence between a substrate and cemented carbide can be increased.
With regard to a cutting tool using a cemented carbide substrate containing WC particles, firstly, Japanese Unexamined Patent Publication No. 2003-127005 describes, as Example 2, an end mill where alternate multilayer films of a TiAlSiN layer and an AlSiN layer are coated on the surface of a cemented carbide substrate obtained by mixing raw material powders including WC powder of middle coarse particles having a mean particle diameter of 5.5 μm and WC powder of fine particles having a mean particle diameter of 0.8 μm, followed by forming and sintering. Secondly, Japanese Unexamined Patent Publication No. 2004-74378 also describes, as Example 2, an end mill where a TiAlNbN layer is coated on the surface of a cemented carbide substrate obtained by mixing raw material powders including WC powder of middle coarse particles having a mean particle diameter of 4.0 μm and WC powder of fine particles having a mean particle diameter of 0.8 μm, followed by forming and sintering. Thirdly, Japanese Unexamined Patent Publication No. 6-220571 describes a cutting tool where a TiCN layer and a TiAlN layer are coated on the surface of a cemented carbide substrate using WC crystals as a main component. The WC crystals contain fine particles A having a particle size of 0.1 to 1.0 μm and coarse particles B having a particle size of 3.0 to 10 μm, and the weight ratio of the fine particles A to the coarse particles B, A/B, is 0.1 to 1.0.
With regard to a cutting tool using a Ti-based cermet substrate, Japanese Unexamined Patent Publication No. 2004-74379 discloses a cutting tool where a TiAlNbN layer is formed on the surface of a cemented carbide or cermet substrate. Japanese Unexamined Patent Publication No. 2005-194573 discloses a cermet cutting tool where a hard film such as a TiAlN is coated on the surface of a cermet substrate having a core structure consisting of a core part and a peripheral part surrounding the core part. The core structure is made up of a first hard phase having a mean particle diameter of 0.1 to 0.8 μm, and a second hard phase having a mean particle diameter of 0.8 to 5 μm, which consists only of a peripheral part.
In high speed machining and the machining of difficult-to-cut materials, particularly hardened steels, which are difficult to be machined with the use of cemented carbide or cermet, a cBN sintered body is employed which is the second hardest (diamond is the hardest) and unsusceptible to the reaction with iron.
For example, Japanese Unexamined Patent Publication No. 59-8679 discloses a cutting tool where a coating layer of TiC, TiN, Al2O3, or the like is formed on the surface of a cBN substrate, and describes that the wear resistance when cutting general steels or cast iron can be improved. Japanese Unexamined Patent Publication No. 8-119774 discloses a cutting tool where a TiAlN coating is formed on the surface of a cBN substrate, and describes that the cutting tool will have a long life even when cutting high hardness difficult-to-cut materials such as hardened steels. Japanese Unexamined Patent Publication No. 2004-345006 discloses a cutting tool having, on the surface of a cBN substrate, a hard coating layer of TiAlSiN composition where the ratio of Ti and Al is changed continuously, and describes that the cutting tool exhibits excellent chipping resistance in high-speed heavy cutting.
Ceramic tools are used when cutting high hardness materials because they are inexpensive and have excellent wear resistance. For example, Japanese Unexamined Patent Publication No. 5-69205 discloses a cutting tool where a coating layer of TiAlN layer or the like is formed on the surface of an Al2O3—TiC based ceramic substrate, and describes that the cutting tool exhibits an excellent cutting performance when cutting high hardness materials. Japanese Unexamined Patent Publication No. 6-91407 describes that the adherence between a ceramic substrate and a coating layer can be increased to improve fracture resistance by forming at least one physical vapor deposition coating layer of TiC, TiN and TiCN on an Al2O3—TiC based ceramics containing 0.1 to 5 weight % of an iron family metal.
However, these coating layers of the related art as described in the above publications are not satisfactory in terms of wear resistance and fracture resistance, and there is a demand for a cutting tool provided with a coating layer more excellent in wear resistance and fracture resistance. It cannot be said that the coated cermet tool disclosed in the above publication No. 2005-194573 are satisfactory in the structural configuration of the cermet substrate and in the coating layer configuration.
In the cutting tools having the cBN substrate, even when any one of the abovementioned TiC, TiN and Al2O3 coating layers, the TiAlN coating layer and the TiAlSiN coating layer is used as the coating layer adhered to the surface of the cBN substrate, cutting performance is insufficient, and a longer life is desired.
Especially, when the cutting tool of the cBN substrate provided with the coating layer is used for hardened steel machining, its tool life cannot be extended for the following reason. That is, in the cutting of the hardened steel, because the surface of a cut article (a material to be cut) has a high hardness, cutting resistance is large and the vicinity of the cutting edge has a high temperature. In particular, temperature is raised in the side surface of the rake face of the cutting edge, which is a passage of chips generated by the cutting. As a result, the coating layer is susceptible of oxidation, and the oxidized coating layer is liable to wear, so that crater wear tends to be advanced in the cutting edge of the tool. Consequently, for example, in light intermittent machining in which continuous machining and intermittent machining are alternately repeated, there is a high risk that the crater wear is advanced during the continuous cutting, and when the intermittent machining is just started, fracture and chipping occur from the advanced crater wear part. This leads to an unsatisfactory tool performance. Further, in order to enhance the hardness of the coating layer itself, it is necessary to increase the residual stress in the coating layer. Because the cBN substrate has high compressive stress in the surface thereof, there is a tendency that when the abovementioned coating layer of the related art is formed on the cBN substrate, this coating layer has a higher residual stress than the case of forming the coating layer on cemented carbide or cermet. As a result, the hardness of the coating layer cannot be enhanced.
Similarly, even when any one of the abovementioned TiAlN coating layer, TiC, TiN or TiCN coating layer and the (TiWSi)N coating layer is used as the coating layer adhered to the surface of the ceramic substrate, cutting performance is insufficient and a longer life is therefore desired.
Especially, when used in severe cutting where the high hardness material cut is subjected to high speed cutting, the tool life cannot be extended for the following reason. That is, in the high speed cutting of the high hardness material, due to the high hardness in the surface of the cut article, the cutting resistance is large and the vicinity of the cutting edge has a high temperature. In particular, the temperature is raised in the side surfaces of the rake face of the cutting edge, which is a passage of chips generated by the cutting. As a result, the coating layer is susceptible of oxidation, and the oxidized coating layer is likely to wear, so that crater wear tends to be advanced in the cutting edge of the tool. Consequently, the crater wear is advanced and, in some cases, there is a high risk that chipping and flaking occur from the crater wear part. This results in an unsatisfactory tool performance. Additionally, because the surface of the ceramic substrate has high hardness and poor toughness, a shock on the coating layer might lead to the fracture at the early stage.